Scr catalytic coating on particulate filters

ABSTRACT

Coated particulate filters and methods for their manufacture disclosed. The particulate filters comprise a honeycomb body having a plurality of porous channel walls defining channels extending from an inlet end to an outlet end. The honeycomb body has an upstream zone having an upstream zone gas permeability and a downstream zone disposed closer to the outlet end than the upstream zone and having a downstream zone gas permeability. SCR catalyst is present in a loading in the downstream zone at localized loading in a range of from about 50 g/L to about 200 g/L such that the upstream zone gas permeability is in a range of about 5 to about 90 times the downstream zone gas permeability.

This application claims the benefit of priority under 35 U.S.C. § 119 of U.S. Provisional Application Ser. No. 62/543,643, filed on Aug. 10, 2017, the content of which is relied upon and incorporated herein by reference in its entirety.

BACKGROUND

This disclosure generally relates to filters for engine exhaust and, more specifically, to ceramic honeycomb filters for reducing NO_(x) and particulate matter in engine exhaust streams such as the exhaust streams of diesel engines, and methods for making the same.

Pollutant exhaust emissions from vehicles operated on either gasoline or diesel fuels have been remediated through implementation of catalyst and particulate filter technologies. It would be desirable to provide DeNO_(x) catalyst configurations on particulate filters that improve catalyst utilization.

SUMMARY

A first aspect of this disclosure pertains to a particulate filter comprising a honeycomb body comprising an inlet end and an outlet end comprising a plurality of porous channel walls extending in an axial direction from the inlet end to the outlet end and defining a honeycomb body axial length L_(a), the plurality of porous channel walls defining channels extending from the inlet end to the outlet end. At least a first set of the channels are plugged, and the honeycomb body further comprises an upstream zone having an upstream zone gas permeability and a downstream zone disposed closer to the outlet end than the upstream zone and having a downstream zone gas permeability. The upstream zone has an upstream zone axial length L_(u) that is less than the honeycomb body axial length L_(a), and the plurality of porous channel walls of the honeycomb body comprises a selective catalytic reduction (SCR) catalyst that promotes selective catalytic reduction of NO_(x), and the SCR catalyst is present in a loading in the downstream zone at localized loading in a range of from about 50 g/L to about 200 g/L such that the upstream zone gas permeability is in a range of about 5 to about 90 times the downstream zone gas permeability.

Another aspect of this disclosure pertains to a lean burn engine exhaust system comprising the particulate filter embodiments described in this disclosure.

Another aspect of this disclosure pertains to a method of manufacturing a catalyzed particulate filter comprising determining a target selective catalytic reduction (SCR) catalyst loading mass for a honeycomb body comprising an inlet end and an outlet end comprising a plurality of porous channel walls extending in an axial direction from the inlet end to the outlet end and defining a honeycomb body axial length L_(a), the plurality of porous channel walls defining channels that permit a flow of gas from the inlet end to the outlet end, wherein at least a first set of the channels are plugged, the target SCR catalyst loading determined based on the honeycomb body axial length L_(a). The method further comprises immersing the outlet end in SCR catalyst slurry and coating the honeycomb body to a length less than axial length L_(a) to provide a coated honeycomb body so that the target SCR catalyst loading mass is contained in less than 75% of the honeycomb body axial length L_(a) of the honeycomb body.

Additional features and advantages will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the embodiments as described herein, including the detailed description which follows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description and the following detailed description are merely exemplary, and are intended to provide an overview or framework to understanding the nature and character of the claims. The accompanying drawings are included to provide a further understanding, and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiment(s), and together with the description serve to explain principles and operation of the various embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

It will be understood that the illustrations are for the purpose of describing particular embodiments and are not intended to limit the disclosure or appended claims thereto. The drawings are not necessarily to scale, and certain features and certain views of the drawings may be shown exaggerated in scale or in schematic in the interest of clarity and conciseness.

FIG. 1 schematically depicts a particulate filter according to one or more embodiments shown and described herein;

FIG. 2 schematically depicts a partial axial cross section of the particulate filter of FIG. 1 according to one embodiment shown and described herein;

FIG. 3 schematically depicts a partial axial cross section of the particulate filter of FIG. 1, according to another embodiment shown and described herein;

FIG. 4 is a graph showing soot loaded pressure drop for various soot loadings for various filters;

FIG. 5 is a graph of pressure drop for filters coated at various lengths;

FIG. 6 is a graph of NO_(x) conversion versus reaction temperature for various filters;

FIG. 7 is a graph of NO_(x) conversion at 250° C. versus coated length of filters; and

FIG. 8 is a graph of NO_(x) conversion versus temperature for various coated filters.

DETAILED DESCRIPTION

Before describing several exemplary embodiments, it is to be understood that the disclosure is not limited to the details of construction or process steps set forth in the following disclosure. The disclosure provided herein is capable of other embodiments and of being practiced or being carried out in various ways.

It is noted that the terms “substantially” and “about” may be utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. These terms are also utilized herein to represent the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue.

As used herein, “DeNO_(x) catalyst” refers to a catalyst that remediates nitrogen oxides (NO_(x)) from an exhaust gas stream, which may emanate from a stationary source or a mobile source such as a lean burn engine of a vehicle. DeNO_(x) catalysts include “lean NO_(x) catalysts,” which selectively promote the reduction of NO_(x) by hydrocarbons, as well as catalysts which promote the selective catalytic reduction (SCR) of NO_(x) by nitrogen compounds, such as ammonia or urea, which are commonly referred to as simply “SCR catalysts.”

Referring now to FIG. 1, a particulate filter is schematically shown according to one or more embodiments shown and described herein. The particulate filter is generally a honeycomb structure comprising a plurality of parallel channels defined by intersecting channel walls. The terms “upstream” and “downstream” will be used herein to describe the relative orientation of zones of the particulate filter. The term “upstream,” as used herein, refers to a zone which is closer in proximity to the inlet end of the particulate filter than the zone to which it is being compared. Similarly, the term “downstream,” as used herein, refers to a zone which is in closer proximity to the outlet end of the particulate filter than the zone to which it is being compared. When a particulate filter is in use, the inlet end of the particulate filter receives exhaust gas from an engine, and the exhaust gas flows through the filter, exiting the filter at the outlet end.

As used herein, the term “permeability” refers to the ability of a fluid to penetrate and flow through the channel walls of the particulate filter. In the embodiments and examples described herein, the permeability of the channel walls can be calculated according to the equation:

${u = {\frac{\kappa}{\mu}\frac{\Delta \; p}{\delta}}},$

where u is the velocity of the fluid through the channel wall in m/s, κ is the permeability of the channel wall in m², μ is the viscosity of the fluid, Δp is the pressure drop across the channel walls in Pascals, and δ is the thickness of the channel wall. Given a particulate filter with a known channel wall thickness and a fluid of a known viscosity, the permeability of the wall can be back calculated after measuring the rate of fluid flow through the channel walls and the pressure drop across the channel walls. Alternatively a porometer can be utilized to determine the permeability of different zones of the particulate filter.

Referring now to FIG. 1, a particulate filter 100 is schematically shown. The particulate filter 100 may be used as a wall-flow filter to filter particulate matter from an exhaust gas stream, such as an exhaust gas stream emitted from a lean burn engine, for example, a diesel engine. The particulate filter 100 may also be used to reduce the content of nitrous oxide compounds (NO_(x)) in the exhaust gas stream. The particulate filter 100 generally comprises a honeycomb body having an inlet end 102 and an outlet end 104 including a plurality of porous channel walls 106 defining a plurality of channels 101 or cells which extend between the inlet end 102 and an outlet end 104 and defining a honeycomb body axial length L_(a). The particulate filter 100 may also include a skin layer 105 surrounding the plurality of channels 101. This skin layer 105 may be extruded during the formation of the channel walls 106 or formed in later processing as an after-applied skin layer, such as by applying a skinning cement to the outer peripheral portion of the channels.

Still referring to FIG. 1 and now the axial cross section of the particulate filter shown in FIGS. 2 and 3, the plurality of channels 101 are generally square in cross section. However, in alternative embodiments, the plurality of channels 101 in the particulate filter may have other cross-sectional configurations, including rectangular, round, oblong, triangular, octagonal, hexagonal, or combinations thereof. For honeycombs utilized in particulate filter applications, certain channels are designated as inlet channels 108 and certain other channels are designated as outlet channels 110. As mentioned above, when disposed downstream from a lean burn engine, exhaust gas enters the particulate filter through the inlet channels and exit the particulate filter through the outlet channels. In some embodiments of the particulate filter 100, at least a first set of channels may be plugged with plugs 112. Generally, the plugs 112 are arranged proximate the ends (i.e., the inlet end 102 or the outlet end 104) of the channels 101. The plugs are generally arranged in a pre-defined pattern, such as in the checkerboard pattern shown in FIG. 1, with every other channel being plugged. The inlet channels 108 may be plugged at or near the outlet end 104, and the outlet channels 110 may be plugged at or near the inlet end 102 on channels not corresponding to the inlet channels, as shown in FIG. 2. Accordingly, each channel may be plugged at or near one end of the particulate filter only.

While FIG. 1 generally depicts a checkerboard plugging pattern, it should be understood that alternative plugging patterns may be used in the honeycomb structure. Further, in some embodiments, a second set of channels may be flow-through channels 109 which are not plugged, as is schematically shown in FIG. 3. In these embodiments, the particulate filter 100 may be referred to as a scalable filter or a partial filter.

In the embodiments described herein, the particulate filter 100 may be formed with a channel density of up to about 600 channels per square inch (cpsi). For example, in some embodiments, the particulate filter 100 may have a channel density in a range from about 100 cpsi to about 600 cpsi. In some other embodiments, the particulate filter 100 may have a channel density in a range from about 100 cpsi to about 400 cpsi or even from about 200 cpsi to about 300 cpsi.

In the embodiments described herein, the channel walls 106 of the particulate filter 100 may have a thickness of greater than about 4 mils (101.6 microns). For example, in some embodiments, the thickness of the channel walls 106 may be in a range from about 4 mils up to about 30 mils (762 microns). In some other embodiments, the thickness of the channel walls 106 may be in a range from about 7 mils (177.8 microns) to about 20 mils (508 microns).

In the embodiments of the particulate filter 100 described herein the channel walls 106 of the particulate filter 100 may have a bare open porosity (i.e., the porosity before any coating is applied to the honeycomb body) % P≥35% prior to the application of any coating to the particulate filter 100. In some embodiments the bare open porosity of the channel walls 106 may be such that 40%≤% P≤70%. In other embodiments, the bare open porosity of the channel walls 106 may be such that 50%≤% P≤67%.

Further, the channel walls 106 of the particulate filter 100 are formed such that the pore distribution in the channel walls 106 has a mean pore size of ≤30 microns prior to the application of any coatings (i.e., bare). For example, in some embodiments, the mean pore size may be ≥8 microns and less than or ≤30 microns. In other embodiments, the mean pore size may be ≥10 microns and less than or ≤30 microns. In other embodiments, the mean pore size may be ≥10 microns and less than or ≤25 microns. In general, particulate filters produced with a mean pore size greater than about 30 microns have reduced filtration efficiency while with particulate filters produced with a mean pore size less than about 8 microns may be difficult to infiltrate the pores with a washcoat containing a catalyst. Accordingly, in one or more embodiments, it is generally desirable to maintain the mean pore size of the channel wall between about 8 microns and about 30 microns.

In the embodiments described herein, the honeycomb body that forms the particulate filter 100 is formed from a ceramic material such as, for example, cordierite, silicon carbide, aluminum oxide, aluminum titanate or any other ceramic material suitable for use in elevated temperature particulate filtration applications. For example, the particulate filter 100 may be formed from cordierite by mixing a batch of ceramic precursor materials which may include constituent materials suitable for producing a ceramic article which predominately comprises a cordierite crystalline phase. In general, the constituent materials suitable for cordierite formation include a combination of inorganic components including talc, a silica-forming source, and an alumina-forming source. The batch composition may additionally comprise clay, such as, for example, kaolin clay. The cordierite precursor batch composition may also contain organic components, such as organic pore formers, which are added to the batch mixture to achieve the desired pore size distribution. For example, the batch composition may comprise a starch which is suitable for use as a pore former and/or other processing aids. Alternatively, the constituent materials may comprise one or more cordierite powders suitable for forming a sintered cordierite honeycomb structure upon firing as well as an organic pore former material.

The batch composition may additionally comprise one or more processing aids such as, for example, a binder and a liquid vehicle, such as water or a suitable solvent. The processing aids are added to the batch mixture to plasticize the batch mixture and to generally improve processing, reduce the drying time, reduce cracking upon firing, and/or aid in producing the desired properties in the honeycomb body. For example, the binder can include an organic binder. Suitable organic binders include water soluble cellulose ether binders such as methylcellulose, hydroxypropyl methylcellulose, methylcellulose derivatives, hydroxyethyl acrylate, polyvinylalcohol, and/or any combinations thereof. Incorporation of the organic binder into the plasticized batch composition allows the plasticized batch composition to be readily extruded. In some embodiments, the batch composition may include one or more optional forming or processing aids such as, for example, a lubricant which assists in the extrusion of the plasticized batch mixture. Exemplary lubricants can include tall oil, sodium stearate or other suitable lubricants.

After the batch of ceramic precursor materials is mixed with the appropriate processing aids, the batch of ceramic precursor materials is extruded and dried to form a green honeycomb body comprising an inlet end and an outlet end with a plurality of channel walls extending between the inlet end and the outlet end. Thereafter, the green honeycomb body is fired according to a firing schedule suitable for producing a fired honeycomb body. At least a first set of the channels of the fired honeycomb body are then plugged in a predefined plugging pattern with a ceramic plugging composition and the fired honeycomb body is again fired to ceram the plugs and secure the plugs in the channels.

Referring now to FIGS. 2 and 3, the particulate filters described herein are formed such that the particulate filter has an upstream zone 120 having an upstream zone gas permeability and a downstream zone 140 disposed closer to the outlet end 104 than the upstream zone 120 and having a downstream zone gas permeability.

In other embodiments (not shown), the upstream zone 120 may be offset from the inlet end 102 of the particulate filter 100. For example, in some embodiments, the upstream zone 120 may be spaced apart from the inlet end 102 of the particulate filter 100 by a separate zone which comprises a coating disposed on the channel walls 106. In general, the axial length L_(u) of the upstream zone 120 is less than the axial length L_(a) of the particulate filter 100 honeycomb body. In some embodiments, the axial length L_(u) of the upstream zone 120 may be 50% or less of the axial length L_(a) of the particulate filter 100. In other embodiments, the axial length L_(u) of the upstream zone 120 may be 33% of the axial length L_(a) of the honeycomb body that forms the particulate filter 100. However, it should be understood that the axial length L_(u) of the upstream zone 120 may be any percentage of the axial length L_(a) of the honeycomb body that forms the particulate filter 100 such that the axial length of the L_(a) of the upstream zone 120 is less than the axial length L_(a) of the honeycomb body that forms the particulate filter 100.

In the embodiments of the particulate filter 100 described herein, the upstream zone 120 is bare. That is, the porous channel walls 106 and/or the pores of the channel walls 106 in the upstream zone 120 do not contain a coating layer which would decrease the permeability of the channel walls 106 to a fluid flowing through the inlet and/or outlet channels. However, it should be understood that, in other embodiments (not shown), the channel walls 106 and/or the pores of the channel walls 106 in the upstream zone 120 may contain a coating layer so long as the coating layer does not decrease the permeability of the channel walls of the upstream zone 120 to less than the permeability of the channel walls in the downstream zone 140. In one or more embodiments, the plurality of porous channel walls of the honeycomb body comprises a selective catalytic reduction (SCR) catalyst that promotes selective catalytic reduction of NO_(x), the SCR catalyst is present in a loading in the downstream zone 140 at a localized loading in a range of from about 50 g/L to about 200 g/L such that the upstream zone 120 gas permeability is in a range of about 5 to about 90 times the downstream zone 140 gas permeability. As used herein, catalyst loading, or total catalyst loading, or total washcoat loading, or washcoat loading, is expressed in grams of catalyst material per external volume of the filter, i.e. the volume calculated from the overall external dimensions of the filter such as overall length and overall diameter. Furthermore, localized loading or localized catalyst loading is defined by the total washcoat loading divided by the fraction F, wherein F is the coated length divided by total length.

The downstream zone 140 is located downstream of the upstream zone 120 and generally extends in an axial direction towards the outlet end 104 of the honeycomb body that forms the particulate filter 100. In one embodiment, the downstream zone 140 is directly adjacent to the upstream zone 120 such that the downstream zone 140 extends from the end of the upstream zone 120 in an axial direction towards the outlet end 104 of the particulate filter 100, as shown in FIGS. 2 and 3. In other embodiments (not shown), the downstream zone 140 may be spaced apart from the upstream zone 120 by one or more intermediate zones positioned between the downstream zone 140 and upstream zone 120. However, it should be understood that, regardless of the spacing between the downstream zone 140 and the upstream zone 120, the downstream zone 140 is always located downstream of the upstream zone 120.

As described above with respect to the upstream zone 120, the downstream zone 140 generally has an axial length L_(d) which is less than the axial length L_(a) of the honeycomb body that forms particulate filter 100. For example, in some embodiments, the axial length L_(d) of the downstream zone 140 may be 50% or more of the axial length L_(u) of the upstream zone 120. In other embodiments, the downstream zone 140 may be 67% of the axial length L_(a) of the honeycomb body that forms the particulate filter 100. In general, the sum of the axial length L_(d) of the downstream zone 140 and the axial length L_(u) of the upstream zone 120 is less than or equal to the axial length L_(a) of the honeycomb body that forms the particulate filter 100.

In one or more embodiments, the upstream zone 120 does not contain any SCR catalyst. In one or more embodiments, honeycomb body that forms the particulate filter 100 comprises SCR catalyst disposed in and/or on the porous channel walls 106 in the upstream zone 120. In one or more embodiments, the honeycomb body comprises SCR catalyst disposed in or on the porous channel walls in the downstream zone 140. As used herein, “in” the porous channel walls refers to SCR catalyst being embedded in the porous walls, or permeating the porous walls of the particulate filter. In other words, SCR catalyst enters the open porosity of the channel walls 106. “On” the porous walls refers to SCR catalyst disposed at the outer surface of the walls, and not embedded within the channel walls 106. In one or more embodiments, SCR catalyst may be disposed both in the channel walls 106 and on the channel walls 106.

In one or more embodiments, the ratio (L_(u)/L_(a)) of the upstream zone 120 axial length L_(u) to the honeycomb body axial length L_(a) is greater than 0 and less than or equal to 0.75, greater than 0.1 and less than or equal to 0.75, greater than 0.15 and less than or equal to 0.75, or greater than 0.15 and less than or equal to 0.60. In one or more embodiments, the downstream zone 140 has a localized catalyst loading in a range of from about 80 g/L to about 200 g/L, from about 100 g/L to about 180 g/L, or from about 120 g/L to about 180 g/L. In one or more embodiments, the upstream zone 120 axial length L_(u) is in a range of from about 50% L_(a) to 80% L_(a), and the downstream zone 140 has a localized catalyst loading in a range of from about 100 g/L to about 180 g/L, from about 100 g/L to about 180 g/L, or from about 120 g/L to about 180 g/L.

In one or more embodiments, the porous channel walls have a porosity in a range of from about 45% to about 75% and a median pore size in a range from 5 micrometers to about 30 micrometers.

Another aspect of this disclosure pertains to a lean burn engine exhaust system comprising the particulate filter of any of the embodiments described above, the lean burn engine exhaust system further including a nitrogenous reductant injector disposed upstream from the particulate filter. Nitrogenous reductant can include ammonia, urea, ammonium carbamate and hydrocarbons (e.g., diesel fuel). The nitrogenous reductant injector can include a reservoir for the reductant, a pump, a pressure regulator and a nozzle to place the nitrogenous reductant in the exhaust gas stream.

In some embodiments, the SCR catalyst may include, without limitation, oxides of base metals such as vanadium, tungsten, molybdenum, ceria, zirconia, and the like as well as mixtures thereof and/or zeolite-based SCR catalysts such as copper-exchanged or iron-exchanged zeolite. In some embodiments, mixtures of all of the above may be used as the SCR catalyst. In one or more embodiments, the SCR catalyst comprises a catalyst material that has a comparative SCR activity that less than a Cu-exchanged SSZ-13 zeolite SCR catalyst. Comparative SCR activity can be determined by evaluating the SCR activity of various SCR catalyst coated on particulate filters having the same properties (porosity, pore size, volume, wall thickness, etc.) with the same catalyst loading, and under the same test conditions (composition of the exhaust gas and space velocity). In one or more embodiments, the SCR catalyst is selected from the group consisting of a Cu-exchanged SAPO-34 molecular sieve, a Cu-exchanged ZSM-5 zeolite, a Cu-exchanged Beta zeolite, and an Fe-exchanged ZSM-5 zeolite. Applicants have determined that disposing an SCR catalyst that has lower comparative SCR activity on a particulate filter as described herein according to one or more embodiments can provide improved NO_(x) removal compared to a particulate filter that is coated along the entire length with the same catalyst loading. Embodiments of the disclosure enable the use of less expensive SCR catalyst materials than Cu-exchanged SSZ-13 SCR catalyst.

In one or embodiments described herein the SCR catalyst is washcoated onto the channel walls 106 of the downstream zone 140 such that the SCR catalyst is on the channel walls 106 of the downstream zone 140, in the pores of the channel walls 106 of the downstream zone 140 (schematically illustrated in FIG. 2), or both on the channel walls 106 of the downstream zone 140 and in the pores of the channel walls 106 of the downstream zone 140 (schematically illustrated in FIG. 3). The SCR catalyst may be deposited in the downstream zone 140 by first forming a slurry of the SCR catalyst in a liquid vehicle, such as water. For example, when the SCR catalyst is a copper exchanged zeolite, the SCR catalyst is mixed with water to forma slurry. The outlet end 104 of the particulate filter 100 is then submerged in the slurry to allow the slurry to infiltrate the particulate filter 100 to a desired depth which, in one embodiment, generally corresponds to the axial length L_(d) of the downstream zone 140. More specifically, the slurry enters the outlet channels 110 and/or flow-through channels 109 of the particulate filter 100 and permeates through the channel walls 106 into adjacent inlet channels 108 via the open pore structure of the channel walls 106 thereby depositing catalyst in the pores of the channel walls 106. In one embodiment, a vacuum system may be attached to the inlet end 102 of the particulate filter 100 when the particulate filter is submerged in the slurry. The vacuum system draws the catalyst upwards and through the channel walls 106. After the particulate filter 100 is removed from the slurry, excess slurry is allowed to drain from the particulate filter 100. In one embodiment, a compressed fluid, such as compressed air, may be injected into the particulate filter 100 to assist in removing the remaining slurry. Thereafter, the particulate filter 100 is dried and calcined. Catalyst loadings are expressed herein in grams/liter after drying and calcining of the coated filter.

Washcoating the particulate filter 100 with a catalyst coating tends to reduce the size of the pores and the porosity in the channel walls 106 as the catalyst is deposited in the pores when the washcoat is removed and/or dried. As a result, the permeability of the washcoated channel walls 106 decreases. In the embodiments described herein, the downstream zone 140 of the particulate filter 100 is washcoated so as to achieve a desired permeability ratio between the upstream zone 120 and the downstream zone 140. In the embodiments described herein, the downstream zone 140 of the particulate filter 100 is washcoated with catalyst such the upstream zone gas permeability is in a range of about 5 to about 90 times, about 6 to about 90 times, about 7 to about 90 times, about 8 to about 90 times, about 9 to about 90 times, about 10 to about 90 times, about 15 to about 90 times, about 20 to about 90 times, about 25 to about 90 times, about 30 to about 90 times, about 35 to about 90 times, about 40 to about 90 times, about 45 to about 90 times, or about 50 to about 90 times the downstream zone gas permeability.

When washcoated in such a way, exhaust gas 200 entering the inlet channels 108 is more prone to pass through the channel walls 106 of the particulate filter 100 in the upstream zone 120 which removes soot from the exhaust gas prior to the exhaust gas being catalytically reacted with the SCR catalyst in the downstream zone 140.

The reduction of NO_(x) compounds in an exhaust gas stream generally involves the reaction of NO_(x) species with a reductant (i.e., CO, H₂, HC, or NH₃) to yield nitrogen and water. For example, ammonia (NH₃) may be injected into the exhaust gas stream to facilitate the reduction of NO_(x) compounds in the exhaust gas stream with the catalyst. The SCR DeNO_(x) reactions proceed according to the following equations:

NO+NH₃+0.25O₂→N₂+1.5H₂O;  a)

NO+NO₂+2NH₃→2N₂+3H₂O; and  b)

0.75NO₂+NH₃→0.875N₂+1.5H₂O,  c)

wherein equation a) is for the standard SCR reaction, equation b) is for the fast SCR reaction, and equation c) is for the NO₂ SCR reaction.

Another aspect of the disclosure pertains to a method of manufacturing a catalyzed particulate filter. The method comprises determining a target selective catalytic reduction (SCR) catalyst loading mass for a honeycomb body that has an inlet end and an outlet end comprising a plurality of porous channel walls extending in an axial direction from the inlet end to the outlet end and defining a honeycomb body axial length L_(a), the plurality of porous channel walls defining channels that permit a flow of gas from the inlet end to the outlet end, wherein at least a first set of the channels are plugged proximate at least one of the inlet end or the outlet end, the target SCR catalyst loading determined based on the honeycomb body axial length L_(a). The method further comprises immersing the outlet end in SCR catalyst slurry and coating the honeycomb body to a length less than axial length L_(a) to provide a coated honeycomb body so that the target SCR catalyst loading mass is contained in less than 75% of the honeycomb body axial length L_(a) of the honeycomb body. In one or more embodiments of the method, the coated honeycomb body has an upstream zone having an upstream zone axial length and an upstream zone gas permeability and a downstream zone having a downstream zone gas permeability, and the porous channel walls are bare in the upstream zone so that the upstream zone gas permeability is in a range of about 5 to about 90 times, about 6 to about 90 times, about 7 to about 90 times, about 8 to about 90 times, about 9 to about 90 times, about 10 to about 90 times, about 15 to about 90 times, about 20 to about 90 times, about 25 to about 90 times, about 30 to about 90 times, about 35 to about 90 times, about 40 to about 90 times, about 45 to about 90 times, or about 50 to about 90 times the downstream zone gas permeability the downstream zone gas permeability.

In one or more embodiments, after immersing the outlet end, the SCR catalyst is present in a loading in the downstream zone in a range of from about 50 g/L to about 200 g/L, about 80 g/L to about 200 g/L, about 100 g/L to about 180 g/L, or about 120 g/L to about 180 g/L. In one or more embodiments, after immersing the outlet end, the target SCR catalyst loading mass is contained in a length ranging of from about 30% L_(a) to 60% L_(a) or in a range of from about 50% L_(a) to 80% L_(a). In some embodiments, after immersing the outlet end, the upstream zone axial length is in a range of from about 50% L_(a) to 80% L_(a) and the downstream zone has a localized catalyst loading in a range of from about 100 g/L to about 180 g/L.

One or more embodiments described herein demonstrate improved diesel particulate filter SCR performance when the SCR catalyst is zone-coated as described herein. Surprisingly, it was found that applying the same catalyst mass to progressively smaller coated lengths resulted in higher NO_(x) conversion and therefore improved catalyst utilization. This provides opportunities to increase the system NO_(x) conversion without adding more catalyst but simply re-arranging the catalyst distribution. Alternatively, if the system NO_(x) conversion is already sufficient, then the catalyst loading could be decreased again by re-arranging the catalyst distribution. A consequence of the zone-coated catalyst arrangement according to one or more embodiments is that portions of the filter remain bare while, with decreasing coated lengths, the permeability of the coated wall decreases.

In one or more embodiments, better catalytic performance can be achieved. If higher NO_(x) conversion is desired for the same fixed amount of washcoat, then catalyst utilization can be increased via re-arrangement of the coating distribution. Cost savings can result. If the NO_(x) conversion is already sufficient, then the washcoat loading can be reduced as a result of improvements to the catalyst utilization via rearrangement of the coating distribution. There can be more flexibility in designing the coated particulate filter. Typically, the only way to improve catalytic activity would be through adding more washcoat or providing a high activity catalyst such as Cu-exchanged SSZ-13, but in one or more embodiments, such high activity catalysts are not necessary. By changing both washcoat loading and coating distribution, one can optimize three important attributes-catalytic conversion, pressure drop, and filtration efficiency. There is more flexibility in designing coated particulate filters since the catalyst utilization is high when the coating is zone-coated. This enables potential catalytic functionality to be added through use of the un-coated portion of the filter. Filtration efficiency typically goes through a minimum with increasing washcoat loading, but by using a combination of bare and highly washcoated walls within the same filter, this minimum in filtration efficiency may be avoided.

In another set of embodiments, a method of manufacturing a catalytic particulate filter is disclosed herein, the method comprising: immersing the outlet end of a honeycomb body comprising porous channel walls in SCR catalyst slurry to a depth less than an axial length L_(a) of the honeycomb body to coat at least a first axial portion of a first plurality of the porous channel walls with SCR catalyst to provide a coated honeycomb body with a target SCR catalyst loading mass which is contained in less than or equal to 75% of the axial length L_(a) of the honeycomb body, wherein the porous channel walls extend in an axial direction from an inlet end to the outlet end.

In some embodiments the SCR catalyst is absent in a region greater than or equal to 25% extending from 5% to 30% of the axial length L_(a) of the honeycomb body.

In some embodiments the honeycomb body comprises a second axial portion wherein the porous channel walls are not exposed to the SCR catalyst slurry, wherein the second axial portion has a permeability which is in a range of about 7 to about 90 times a permeability of the first axial portion.

In some embodiments the SCR catalyst is present in a loading in the first axial portion in a range of from about 50 g/L to about 200 g/L.

In some embodiments the target SCR catalyst loading mass is contained in the first axial portion in a range of from about 30% L_(a) to 60% L_(a).

In some embodiments the target SCR catalyst loading mass is contained in a range of from about 50% L_(a) to 80% L_(a).

In some embodiments the first axial portion has a localized catalyst loading in a range of from about 80 g/L to about 200 g/L.

In some embodiments the first axial portion has a localized catalyst loading in a range of from about 100 g/L to about 180 g/L.

In some embodiments the first axial portion has a localized catalyst loading in a range of from about 120 g/L to about 180 g/L.

In some embodiments the second axial portion is in a range of from about 50% L_(a) to 80% L_(a) and the first axial portion has a localized catalyst loading in a range of from about 100 g/L to about 180 g/L.

In some embodiments the ratio (L_(u)/L_(a)) of the upstream zone axial length L_(u) to the honeycomb body axial length L_(a) is greater than 0 and less than or equal to 0.75.

In some embodiments the ratio (L_(u)/L_(a)) of the upstream zone axial length L_(u) to the honeycomb body axial length L_(a) is greater than 0.1 and less than or equal to 0.75.

In some embodiments the ratio (L_(u)/L_(a)) of the upstream zone axial length L_(u) to the honeycomb body axial length L_(a) is greater than 0.15 and less than 0.75.

In some embodiments the ratio (L_(u)/L_(a)) of the upstream zone axial length L_(u) to the honeycomb body axial length L_(a) is greater than 0.15 and less than 0.60.

EXAMPLES

Four samples were made with different catalyst distributions on identical particulate filters 6 inches in axial length. The target washcoat loading was 75 g/L for all four samples with progressively smaller coated lengths Comparative Example 1 (labeled #7 in the Figures) had a washcoat loading evenly distributed along the axial length of the filter for a loading of 80 g/L. Example 1 (labeled #5 in the figures) had a 1 inch upstream zone that was bare or uncoated, and the downstream zone containing SCR catalyst was 5 inches long. The SCR catalyst loading was 75 g/L for the filter. Example 2 (labeled 6A in the Figures) had a loading of 77 g/L, and the upstream zone was 2 inches in length and was bare or uncoated, while the downstream zone was coated and 4 inches long. Example 3 (labeled 12A in the Figures) had a loading of 79 g/L, with the bare or uncoated upstream zone 3 inches in length and the coated downstream zone 79 g/L.

All samples were coated with Cu-exchanged SAPO from Zeolyst International. The filters were a high porosity filter (HPF) cordierite composition. The filters were coated using an immersion process through either the outlet or inlet channels to the specified length. The excess slurry was removed using coupled vacuum with the filter in the same orientation it was during coating. For partial length coatings, the filter is lowered an appropriate distance to achieve the desired coated length. The slurry flows into the filter through channel and wall capillary forces as well as hydrostatic pressure. The immersion depth is correlated to the coated length as one would expect, although it is not a 1:1 correlation as the slurry will wick into the porous walls ahead of the channel liquid front. In order to increase the local washcoat loading, the slurry solids loading has to be increased. Achieving 180 g/L for the 3″ coated filter is right at the edge of what we can process with immersion as the slurry introduction method due to the increased viscosity as the solids loadings increase. Going any higher in solids loading leads to slip-casting on the endface due to the presence of the channel plugs which act as a sink for the water phase and lead to solids deposition on the endface often entirely blocking the channels from taking in any slurry.

The filters were dried (100° C.) and calcined (550° C./3 hr) before NO_(x) SCR testing. Table 1 shows more details for the various samples. Example 4 (labeled as 7A) was coated similar to Example 1, except Example 4 was coated from the inlet. Example 5 (labeled as 9A) was coated similar to Example 2, except Example 5 was coated from the inlet end. Soot loaded pressure drop performance was evaluated at 26 scfm air at room temperature with commercially available soot Printex U, using filters of 2″ diameter, 6″ length, 350 cells per square inch (cpsi) honeycomb structure having 12 mil (305 micrometer) wall thickness.

TABLE 1 Coated Bare Coated Weight Global Local Coating length Part ID wt [g] wt [g] gain [g] WCL [g/L] WCL [g/L] direction [inches] 7 (Comp Ex. 1) 141.87 165.88 24.01 79.89 79.89 Outlet 6 5 (Ex. 1) 141.89 164.55 22.66 75.40 92.35 Outlet 5 6A (Ex. 2) 140.35 163.48 23.13 76.72 113.28 Outlet 4 12A (Ex. 3) 139.27 164.24 24.97 81.10 181.1 Outlet 3 7A (Ex. 4) 141.26 164.18 22.92 76.30 76.30 Inlet 6 9A (Ex. 5) 139.98 162.90 22.92 76.60 89.35 Inlet 5

For Comparative Example 1 through Example 3, the localized washcoat loading is increased from about 80 to 92 to 113 to 181 g/L, indicating a more than doubling of the localized washcoat loading for the 3″ coated sample compared with Comparative Example 1. Although a target coating level of 75 g/L was attempted on each sample, it can be seen that in few cases the target level was exceeded. The last two samples included in this table (Examples 4 and 5) are coated in the opposite side of the sample, but further data on these samples was obtained.

Pressure Drop Testing was accomplished by evaluating the pressure drop of soot loaded filters having 2″ diameter, 6″ length, 350 cells per square inch (cpsi) honeycomb structure with walls of 12 mil (305 micrometer) thickness at 26 scfm air at room temperature with commercially available soot Printex U.

FIG. 4 shows pressure drop response measured as function soot loading on full and partially coated filters, as well as a bare filter, labelled “bare OSV” control sample. The uncoated (control sample) high porosity filter (diamond) showed relatively no knee in the soot loaded pressure drop test. Next, the coated soot loaded pressure drop for the 5″ (Example 1) (square) and 6″ (Comparative Example 1) (triangle) coated length via the outlet channels is similar. A knee around 0.5 g/L soot is observed for both filters. The 3″ Example 3 (circle) coated length via the outlet channels filter has the highest soot loaded pressure drop. The knee observed in the other coated samples is not apparent at this coated length. The 4″ coated filter (Example 2) used in catalytic studies unfortunately broke there was no pressure drop data available. The impact of coating is similar for the 5″ and full length coated filters while the pressure drop for the 3″ coated filter is clearly a step higher. FIG. 4 breaks out pressure drop for selected conditions, namely clean (no soot) and at 2 g/L and 4 g/L soot loading.

FIG. 5 shows the increase in pressure drop from clean (0 g/L soot; light shaded bars) to the forecasted values for 2 g/L (dark shaded bars) and 4 g/L (medium shaded bars) soot loading. The relative percent changes in the pressure drop, based on the clean pressure drop, are shown at the top of each bar.

NO_(x) reduction testing was evaluated at space velocity 70,000 h⁻¹, 500 ppm NO_(x), and 500 ppm NH₃, at various temperatures as seen in the FIGS.

FIG. 6 shows the SCR catalytic performance results collected using a laboratory bench reactor on full (6″ length) and partially (3-4″ length) coated HPF filters. Surprisingly, the NO_(x) activity increased for samples with decreased coating length. Given that the washcoat loadings were all in the range 75-80 g/L, the increase in conversion indicates higher catalyst utilization. FIG. 6 clearly shows a disadvantage with the full length coating as it has the lowest NO_(x) conversion of the filters tested. Furthermore, and counter-intuitively, the filters with modified coating distributions show higher NO_(x) conversion with decreasing coated length. Repeat measurements at all temperatures on 3 of the 4 samples showed variability from 2-3% absolute NO_(x) conversion indicating that the differences between the samples are large enough to reflect real differences between the filter's performance. FIG. 7 shows this trend more succinctly with the conversion at 250° C. obtained from FIG. 6 as a function of the coated length. It is expected that shorter coated lengths with the same washcoat amount for the entire part would show that the conversion goes through a maximum simply because at some point the washcoat would be packed so tightly that it would hinder gas transport.

FIG. 8 shows NO_(x) conversion as a function of temperature for both forward and reverse flow orientations for full and partially-coated filters. The 4″ coated part broke, and no reverse flow data could be collected. The results clearly show similar conversion in all cases, indicating, in particular for the partially coated filters, that axial non-uniformities in temperature for example are not driving the partial coating utilization benefits described herein. FIG. 8 illustrates that is not the case, as NO_(x) conversions are compared as a function of temperature for forward and reverse flow through the full and partial coated filters. It is clear that the conversion is not significantly impacted by the filter orientation in the reactor indicating that thermal non-uniformity is not driving the results shown here, but instead that we are in fact increasing the catalyst utilization.

The magnitude of the NO_(x) conversion differences observed herein may be more applicable to lower activity SCR catalysts, than other catalysts such as Cu-exchanged SSZ-13, where the inherent catalyst utilization may already be high. Embodiments described herein could provide cost savings for emerging markets where state-of-the-art NO_(x) reduction is not required. Thus, embodiments provide less expensive, less robust SCR catalyst that are distributed in the filter such that sufficient NO_(x) conversion is achieved to meet regulations while keeping cost, a critical factor in emerging markets, to a minimum.

It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the spirit or scope of the disclosure. 

1. A particulate filter comprising: a honeycomb body comprising an inlet end and an outlet end comprising a plurality of porous channel walls extending in an axial direction from the inlet end to the outlet end and defining a honeycomb body axial length L_(a), the plurality of porous channel walls defining channels extending from the inlet end to the outlet end, wherein at least a first set of the channels are plugged, the honeycomb body further comprising an upstream zone having an upstream zone gas permeability and a downstream zone disposed closer to the outlet end than the upstream zone and having a downstream zone gas permeability, and the upstream zone has an upstream zone axial length L_(u) that is less than the honeycomb body axial length L_(a), and the plurality of porous channel walls of the honeycomb body comprises a selective catalytic reduction (SCR) catalyst that promotes selective catalytic reduction of NO_(x), and the SCR catalyst is present in a loading in the downstream zone at localized loading in a range of from about 50 g/L to about 200 g/L such that the upstream zone gas permeability is in a range of about 5 to about 90 times the downstream zone gas permeability.
 2. The particulate filter of claim 1, wherein the upstream zone does not contain any SCR catalyst.
 3. The particulate filter of claim 1, wherein the honeycomb body comprises SCR catalyst disposed in or on the porous channel walls in the upstream zone.
 4. The particulate filter of claim 1, wherein the honeycomb body comprises SCR catalyst disposed in or on the porous channel walls in the downstream zone.
 5. The particulate filter of claim 1, wherein the first set of the channels are plugged proximate at least one of the inlet end or the outlet end.
 6. The particulate filter of claim 1, wherein the ratio (L_(u)/L_(a)) of the upstream zone axial length L_(u) to the honeycomb body axial length L_(a) is greater than 0 and less than or equal to 0.75.
 7. The particulate filter of claim 1, wherein the ratio (L_(u)/L_(a)) of the upstream zone axial length L_(u) to the honeycomb body axial length L_(a) is greater than 0.1 and less than or equal to 0.75.
 8. The particulate filter of claim 1, wherein the ratio (L_(u)/L_(a)) of the upstream zone axial length L_(u) to the honeycomb body axial length L_(a) is greater than 0.15 and less than or equal to 0.75.
 9. The particulate filter of claim 1, wherein the ratio (L_(u)/L_(a)) of the upstream zone axial length L_(u) to the honeycomb body axial length L_(a) is greater than 0.15 and less than 0.60.
 10. The particulate filter of claim 1, wherein the downstream zone has a localized catalyst loading in a range of from about 80 g/L to about 200 g/L.
 11. The particulate filter of claim 1, wherein the downstream zone has a localized catalyst loading in a range of from about 100 g/L to about 180 g/L.
 12. The particulate filter of claim 1, wherein the downstream zone has a localized catalyst loading in a range of from about 120 g/L to about 180 g/L.
 13. The particulate filter of claim 1, wherein the upstream zone axial length L_(u) is in a range of from about 50% L_(a) to 80% L_(a) and the downstream zone has a localized catalyst loading in a range of from about 100 g/L to about 180 g/L.
 14. The particulate filter of claim 1, wherein the porous channel walls have a porosity in a range of from about 45% to about 75% and a median pore size in a range from 5 micrometers to about 30 micrometers.
 15. A lean burn engine exhaust system comprising the particulate filter of claim 1, further comprising a nitrogenous reductant injector disposed upstream from the particulate filter. 16.-25. (canceled)
 26. A method of manufacturing a catalytic particulate filter comprising: immersing an outlet end of a honeycomb body comprising porous channel walls in SCR catalyst slurry to a depth less than an axial length L_(a) of the honeycomb body to coat at least a first axial portion of a first plurality of the porous channel walls with SCR catalyst to provide a coated honeycomb body with a target SCR catalyst loading mass which is contained in less than or equal to 75% of the axial length L_(a) of the honeycomb body, wherein the porous channel walls extend in an axial direction from an inlet end to the outlet end.
 27. The method of claim 26, wherein the SCR catalyst is absent in a region greater than or equal to 25% extending from 5% to 30% of the axial length L_(a) of the honeycomb body.
 28. The method of claim 26, wherein the honeycomb body comprises a second axial portion wherein the porous channel walls are not exposed to the SCR catalyst slurry, wherein the second axial portion has a permeability which is in a range of about 7 to about 90 times a permeability of the first axial portion.
 29. The method of claim 26, wherein the SCR catalyst is present in a loading in the first axial portion in a range of from about 50 g/L to about 200 g/L.
 30. The method of claim 26, wherein, the target SCR catalyst loading mass is contained in the first axial portion in a range of from about 30% L_(a) to 60% L_(a). 31.-39. (canceled) 